Stroking the B3-3

In my last post, I mentioned installing a shorter piston in my B3-3 to increase power. I was originally going to use a piston from a B4-2, but that turned out to be too small. Looks like I’m going to have to modify my original piston.

Piston air rifles are similar to car engines. Common techniques to improve power with car engines are boring and stroking. Boring involves making the cylinders a tad larger. With multiple cylinders, this can really add up. Our rifle has a single cylinder and not enough meat to remove much. Stroking is going to be our best bet. More air will be ahead of the piston and the piston will generate more speed and momentum, both increasing power.

A shorter piston gives a longer power stroke. The original stroke length was only 58mm, much too short to produce any real power. In fact, it produces 7.73 fpe at the muzzle with RWS Superpoints. Let’s see if we can shorten the piston, increasing the stroke, and gain some power…

I noticed that the head of the piston was attached to the skirt by means of pins. I drilled those out and expanded the skirt by hammering a wedge down the back seam. With a bit of prying and wiggling, the head popped free from the skirt:

Time for modification
Time for modification

Take a close look at the skirt on the right. You’ll see a collar that fits in the rear groove of the piston. I cut the skirt right behind that collar. The head was then placed back into the skirt. I placed the whole assembly on a small anvil and hammered the skirt carefully around the head. Here’s what it looked like at that point:

Cut down piston by the compression chamber
Cut down piston by the compression chamber

To attach the head to the skirt, I drilled holes in the skirt above the rear groove in the head. I then welded them together, concentrating the heat into the head, and letting the puddle bond with the skirt. The same technique was used on the top and bottom where the seam and cocking slot exposed the head.  In this way, the head and skirt have become one nearly seamless piece, much stronger than the original pinned design.

I then rounded the piston on a belt sander. It fits the compression chamber beautifully. Several holes were drilled along the body of the skirt to further lighten it. The cocking slot was ground into the head to give the necessary clearance for the longer stroke. Here’s what it is looking like:

Nearly completed piston
Nearly completed piston

Ignore the plastic on the rear for now. I cleaned everything up and added a light coat of lube. There was some minor adjustments I had to make to the cocking linkage. To test the fit and function, I installed the spring with almost no preload. The results were immediately impressive. The RWS Superpoints went from averaging 490 fps with the spring nearly coilbound to 525 fps with almost no preload. This is an increase of over a foot pound, with a huge decrease in cocking effort. Stroke length went from 58 mm to 70 mm.

Of course, I’m not content with that. I calculated a potential muzzle velocity of 588 fps and began increasing spring preload in an attempt to get there. I got to 545 fps and then ran into a bit of trouble. There was too much play between the rear of the skirt and the body of the action. Under heavier spring tension, the sear would not stay engaged, and the rifle would attempt to fire as the cocking lever was brought forward. Remember that plastic in the last picture? That was a lame attempt to fix that issue. It fixed that issue, but also acts as a brake.

Moral of the story – always remember these rifles are known as finger chompers! Keep your fingers out of the breech unless you have a firm grip on the cocking arm with your other hand. When returning the lever, make sure your hands are not between the lever and barrel. I observe these rules and my fingers and hands are intact.

To fix this issue, I’m going to attempt heat forming HDPE plastic to fashion buttons. This will provide a low friction surface between the skirt and action wall, while keeping everything tight and centered.

Author: Geo

I've enjoyed shooting and hunting with airguns since my early teen years. For well over a decade, I have shared my passion for airguns on this website.

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